A helicopter rotor blade functions as a rotating airfoil that or wing that is shaped to provide aerodynamic lift. The rotor blades can be subjected to a relatively complex set of extreme aerodynamic forces that vary continually during flight.
Helicopter rotor blades typically include a spar that extends from the root of the rotor blade to its tip. The spar is a major structural element that provides the rotor blade with most or all the structural strength required to carry its operational load. A typical rotor blade also incorporates a skin mounted on the spar. The skin forms an airfoil having a leading edge, a trailing edge, and upper and lower surfaces. Integral counterweights and balance-weight fittings can be incorporated into the spar during fabrication, or added to the blade assembly after fabrication of the spar.
The spar of a helicopter rotor blade is typically an elongated structure around which the rest of the rotor blade is formed. Many spars for helicopter blades incorporate a slight twist about their longitudinal axes to help achieve optimum aerodynamic performance. For example, spars having elliptical cross sections may incorporate a twist of up to approximately ten degrees between the root and tip thereof. In addition, the cross section of the spar may vary along its length to meet a variety of aerodynamic and structural loading parameters. For example, the cross sectional geometry of a spar may have a length-wise variation to accommodate different airfoil shapes along the length of the rotor that help achieve maximum aerodynamic performance.
Spars for helicopter rotor blades and other aerostructures can be formed from high-strength metals such as titanium and aluminum alloys. For example, titanium has been formed into relatively long tubular spars with a certain amount of twist on a routine basis.
Spars can also be formed from composite materials. The combination of high strength and light weight make the use of composite materials a popular choice for manufacturing the spar, as well as the remainder of the rotor blade.
A composite spar can be fabricated by applying layers or plies of uncured composite material to the surface of an elongated female mold having the desired distribution of contour and twist. Each layer of composite material can be formed from layers of continuous fiber reinforcements impregnated with a viscous, sticky epoxy resin. The partially-formed perform or laminate may be moved onto an undersized inflatable pressure bag or bladder supported internally by a stiff mandrel covered with styrofoam or another suitable supporting material. The pre-form can then be completed, or “closed-out,” by adding additional layers of the composite material around the bladder, and around the exterior of the pre-form.
The bladder and foam-covered mandrel assembly has to be suitably undersized to provide clearance between the bladder and the interior surfaces, or inside mold line of unconsolidated pre-form, thus ensuring that the closed-out, wrapped pre-form will be undersized and transferable to a mold without interference.
The unconsolidated pre-form contains air that becomes trapped between layers of the pre-form during the hand lay-up process, resulting in extra bulk or thickness in the layed-up material. Moreover, the undersized bladder/mandrel/foam assembly can result in the presence of air between the bladder/mandrel/foam assembly and the inside mold line of the pre-form.
The pre-form can subsequently be placed in the mold. The composite material from which the pre-form is formed can be cured at an elevated temperature while being compacted or consolidated by the inflated bladder which urges composite material with molten resin outward against the interior surfaces of the mold.
Molten resin, air, and other gases can potentially become trapped in voids within the composite material during the curing process. In particular, as the inflated bladder expands it first contacts the high points on the upper and lower walls of the inside mold line of the pre-form, squeezing the molten resin and trapped air and other gases into those regions where the bladder bridges across (i) the low points on the walls the inside mold line of the pre-form, and/or (ii) over the inside of the corners of the pre-form.
For example, FIG. 15 depicts a cross section of a portion of a finished spar 102 formed using a conventional manufacturing process. The figure depicts pockets or voids 104 in the corners of the finished spar 102. These voids 104 are believed to have resulted from the presence of resin and gases trapped between the layers of composite materials during the curing and consolidation process, as shown in FIG. 16. FIG. 16 depicts the spar 102 being formed from a pre-form 10a which is being consolidated by a pressure bag or bladder 20a. The bladder 20a is depicted as pressing the pre-form 10a against interior surfaces of a mold 12a. The voids 104 tend to form in the corner regions of the pre-form 10a, where abrupt changes in shape that can make it difficult for adjacent layers of composite material in the pre-form 10a to contact each other. Voids 104 can form in other areas of the pre-form 10a as well.
The presence of the pockets or voids 104 between the layers of the composite material during the curing and consolidation process can adversely affect the ability of the bladder to exert a substantially uniform pressure on the composite material, which in turn can potentially compromise the compaction and consolidation process and result in localized areas of reduced strength in the finished spar 102. The presence of voids 104 between adjacent layers the pre-form 10a can also result in the voids 104 being permanently formed into the finished spar 102 as depicted in FIG. 15. Voids such as the void 104 can result in further localized reductions in strength in the finished spar 102.
Moreover, excess resins, and air and other gases can become trapped between the exterior surface of the bladder 20a and the interior surface of the pre-form 10a, and between the exterior surface of the pre-form 10a and the interior surface of the mold 12a during the curing and consolidation process, as shown in FIG. 16. This is due, in part, to the requirement that the un-inflated bladder 20a, and the underlying structure that supports the bladder 20a must necessarily be undersized in relation to the interior of the pre-form 10a to permit the bladder 20a and its supporting to be positioned within the pre-form 10a. When the bladder 20a is inflated during the consolidation process, this layer of air can become compressed between the bladder 20a and the interior surface of the pre-form 10a due to the force exerted by the bladder 20a, and the reactive force exerted by the pre-form 10a and the adjacent walls of the mold 12a. For example, the air between the bladder 20a and the pre-form 10a can be compressed from a pressure of approximately atmospheric, to a pressure of approximately 100 psi during consolidation of the pre-form.
The layer of trapped resin and air and other gases can prevent the pre-form 10a from being uniformly consolidated. For example, the trapped air will tend to migrate to “low spots” on the pre-form 10a as the pre-form is consolidated. The low spots typically occur at or near the corners of the pre-form 10a, and at other contoured areas at which the interior surface of the pre-form 10a undergoes an abrupt change in shape. The internal tension within the bladder 20a can keep the bladder from expanding to follow these contours. This results in “bridging” as shown in FIG. 16, and the formation of pockets or voids 105 between the bladder 20a and the pre-form 10a. Moreover, bridging of the pre-form 10a can result in voids 105 between the corner regions of the pre-form 10a and the adjacent surfaces of the mold 12a as also depicted in FIG. 16. The voids 105 can prevent the adjacent portions of the pre-form 10a from being properly consolidated.
Bridging can be particularly troublesome in the corner regions of the pre-form 10a due to the relatively abrupt changes in the geometry of the pre-form 10a at those locations, and the natural tendency of the inflated bladder 20a to assume a circular or spherical shape when it is not confined by an external force such as the contact force between the bladder 20a and the pre-form 10a. For example, as shown in FIG. 17, the inflated bladder tends 20a to contact the nearly flat upper and lower walls of the pre-form 10a, and makes point contact with the center region of the back wall or web of the pre-form 10a. The bladder 20a bridges the corner regions between the back wall and the upper and lower walls. Thus, the bladder 20a does not exert pressure on the corner regions (or on the underlying portions of the pre-form 10a).
As the resin in the pre-form 10a is heated and becomes fluid it tends to migrate to the lower-pressure regions inside the internal voids 104 within the pre-form 10a, and into the gap and voids 105 between the pre-form 10a and the bladder 20a can be forced toward the voids 105 formed at the corner regions of the pre-form 10a and at other low spots at which direct contact between the bladder 20a and the pre-form 10a does not occur. The hydrostatic pressure of the molten resin and the compressed air and other gases in these regions can prevent the bladder from exerting pressure on the underlying portions of the pre-form 10a. This can prevent those portions of the pre-form 10a from being properly consolidated, which in turn can result in localized reductions in strength in the finished spar 102.